1. Field of the Invention
The present invention relates to an information transmission system and more particularly to an information transmission system with a record/reproducing device.
2. Related Background Art
There are already known a digital video tape recorder for recording and reproducing digital video signals, and a system for transmitting and receiving digital video signals.
Consequently a system for recording a video signal in a digital video tape recorder and transmitting the reproduced video signal after encoding can be realized by the combination of known technologies, as shown in FIG. 1. An analog video signal supplied to an input terminal 1 is entered into a video tape recorder 2. An analog video signal released therefrom is supplied to an encoder (transmitter) 3 for encoding and band compression. The output of the encoder 3 is supplied through a transmission channel 30 to a decoder 4 constituting a receiver, for signal expansion and D/A conversion. Thus, the original analog video signal is obtained at an output terminal 5.
In the video tape recorder 2, there are provided an A/D converter 12, a band compression circuit 14, an electromagnetic converting unit 10, an expansion circuit 22 for effecting an expansion inverse to the process of the band compression circuit 22, and a D/A converter 24. In the encoder 3 there are provided an A/D converter 32, and a band compression circuit 34 for transmission. In the decoder 4 there are provided an expansion circuit 42 for effecting an expansion inverse to the process conducted in the band compression circuit 34, and a D/A converter 44.
As an application of the above-mentioned signal transmission system, there can be conceived a system in which plural receivers with decoders can receive desired programs from plural transmitting video tape recorders (image sources) through an exchange. In such a system, the transmitting side has conventionally been composed of plural transmitters, each composed of the video tape recorder 2 and the encoder 3, and the outputs of said transmitters have been connected to said exchange.
However, in the above-explained system, an A/D conversion, and signal compression and expansion are conducted in the video tape recorder, and also in front of and behind the transmission channel 30. Such repeated processes lead to a complication in the structure, thus resulting in a longer time in assembly, a lowered reliability, an elevated cost, a larger weight and a difficulty in making the equipment compact. These drawbacks become more marked when plural transmitters and plural receivers are connected through the exchange, as there is required many groups of equipment.
FIG. 2 shows a similar system, in which the input and output of the video tape recorder, the input of the encoder and the output of the decoder are digital video signals, and in which a function of error correction in the codes is further added.
In FIG. 2, a digital video signal is subjected to a band compression in a band compression circuit 14, and is supplied to an error correction code (ECC) adding circuit 16, which adds an error correction code for detecting and correcting errors resulting from recording and reproduction. The output of the ECC adding circuit 16 is supplied to an error correction circuit 26 through an electromagnetic conversion unit 10 comprising magnetic heads and a magnetic recording medium. Said error correction circuit 26 corrects the errors caused during recording and reproduction in the electromagnetic conversion unit, utilizing the error correction code added in the ECC adding circuit 16. If correction is not possible, the error correction circuit 26 releases an interpolation flag signal 27. An expansion circuit 22 applies, to the output of the error correction circuit 26, an expansion inverse to the band compression in the band compression circuit 14, and an interpolation circuit 28 effects interpolation on the data for which correction is not possible, according to the interpolation flag 27 s et by the error correction circuit 26. The circuits 14 to 28 correspond to a digital video tape recorder 2, and the output of the interpolation circuit 28 is digital video signal.
The output of the interpolation circuit 28 is supplied to a band compression circuit 34 for transmission and is band compressed therein. Then, the error correction code for transmission is added in an ECC adding circuit 36. The band compression circuit 34 and the ECC adding circuit 36 correspond to the encoder 3.
The output data of the ECC adding circuit 36 are supplied, through a transmission channel 30, to an error correction circuit 46 of the receiving side, which corrects the error in the transmission according to the error correction code added by the ECC adding circuit 36. If the error is uncorrectable, the error correction circuit 46 releases an interpolation flag signal 47. An expansion circuit 42 applies, to the output of the error correction circuit 46, an expansion corresponding to the band compression in the band compression circuit 34, and an interpolation circuit 48 effects an interpolation on the uncorrectable data, according to the interpolation flag signal 47 from the error correction circuit 46. The circuits 46 to 48 correspond to the decoder 4, and the output of the interpolation circuit 48 is digital video signal.
In addition to the drawbacks associated with the system shown in FIG. 1, the system shown in FIG. 2 has a drawback of requiring two interpolation circuits 28, 48, as the interpolations for the errors in the electromagnetic conversion unit 10 and in the transmission channel 30 are conducted separately. Also, for interpolation in the interpolation circuit 28, the compressed data are expanded in the expansion circuit 22, and are again compressed in the band compression circuit 34 for data transmission.
Therefore the system of FIG. 2 is difficult to make compact as efficient utilization of circuits is not at all provided. Also, the repeated compressions and expansions of the video signal may result in deterioration of image quality. The possibility of such deterioration in image quality is high if the band compression method for recording is different from that for transmission, for example if a sub-sampling is conducted by a field offset method while the other is conducted by a line offset method, or if the quantizers for differential quantization have different set values.
FIG. 3 shows a similar system in which band compression associated with high-efficiency encoding is not conducted. Such a system is adapted for use as an audio processing circuit in a system provided with a video processing circuit and an audio processing circuit. In the system shown in FIG. 3, an analog audio signal supplied to a terminal 1 is digitized in an A/D converter 12, and an error correction signal for correcting the error in recording and reproduction is added by an error correction code adding circuit 16. Then, said signal is modulated in a modulating circuit 18 and is recorded on a recording medium T such as a video tape, by means of a magnetic head H.sub.R (a rotary magnetic head in the case of a video tape recorder. Another magnetic head H.sub.P (magnetic head H.sub.R may also be used for this purpose) reproduces the magnetic signal from the recording medium T, and said signal is demodulated by a demodulating circuit 29. An error correction circuit 26 corrects the error in recording and reproduction, by the error correction code added by the error correction code adding circuit 16.
In the transmission of the thus reproduced signal, an error correction code adding circuit 36 adds, to the output of the error correction circuit 26, an error correction code for correction the error in the transmission, and a modulating circuit 38 effects modulation for transmission, and the signal is supplied to the transmission channel 30. A demodulating circuit 49 receives and demodulates the data transmitted through the channel 30, and an error correction circuit 48 corrects the error in transmission, by the error correction code added by the error correction code adding circuit 36. A D/A converter 44 converts the output digital signal of the error correction circuit 46 into analog signal.
Also in such system, the error corrections conducted in two stages increase the magnitude of circuitry and the cost of the system.